@Bob Ludwick...<blockquote> I don’t know about weeds, since few commercial greenhouse operators produce them. And I have no idea how much it would cost</blockquote>Well, I don't know that much, although I <b>do</b> know a fair amount about evolution, and I've spent some time with peer-reviewed science regarding weeds. I see a risk. <b>Not a certainty</b>, but a risk that an outbreak of "sleeper weeds" could impact agriculture. As well as a more distributed risk of other types of catastrophic (in the mathematical sense) eco-reorganizations. Such things could potentially have major impacts on our (world-wide) economic system, perhaps even to the same extent as a collapse caused by dramatic rises in energy prices. Both risks should, IMO, be reduced, by keeping the price of energy low, while pursuing "low-regrets" options to minimize the risk from fossil carbon. As Steven Mosher has pointed out, any (unquantified) benefit to crops from increased atmospheric pCO2 can be balanced by a(n unquantified) risk from newly evolved "sleeper weeds". This was, of course, implicit in my original statement that "weeds are plants".
&@DocMartyn...
More greenhouses are a good idea, since it would probably reduce humanity's agricultural footprint on the planet, as well as making an <b>enormous</b> contribution to adaptability. And not only regular greenhouses, but especially <a href="http://en.wikipedia.org/wiki/Seawater_greenhouse" rel="nofollow">seawater greenhouses</a>.
